1. Field of the Invention
The present invention relates to a process for producing a polycarbonate, and more particularly to a process for efficiently producing a polycarbonate excellent in transparency, heat resistance, and water resistance.
2Description of the Related Arts
Generally, in order to produce a polycarbonate (hereinafter referred to as "PC"), there have been known the interfacial method as a method of directly reacting an aromatic dihydroxy compound such as bisphenol A with phosgene, and the melt method as a method of transesterifying an aromatic dihydroxy compound such as bisphenol A with a carbonic acid ester such as diphenyl carbonate both in a molten state.
However, the interfacial method has a problem in that a poisonous phosgene must be used, the production equipment is corroded by a chlorine-containing compound such as hydrochloric acid or sodium chloride which is by-produced in the reaction, and that impurities such as sodium hydroxide contaminating into the resin are difficult to separate. These impurities have ill effects on the physical properties of the polymer.
On the other hand, the melt method has an advantage of producing PC at a lower cost than in the interfacial method, but is involved in a serious disadvantage of ineviatble coloring of the resulting resin since the reaction usually continues for a long period of time at so high a temperature of 280.degree. C. to 310.degree. C.
To reduce the coloring in the melting method, various improvements have been proposed. Japanese Patent Publication No. 39972/1986 and Japanese Patent Application Laid-Open No. 223036/1988, for instance, disclose a method using a particular catalyst. Japanese Patent Application Laid-Open No. 151236/1986 and Japanese Patent Application Laid-Open No. 158719/1987 disclose methods wherein antioxidants are added in the latter stage of reaction. Moreover, Japanese Patent Application Laid-Open No. 62522/1986 and Japanese Patent Application Laid-Open No. 153925 disclose improvements in processes, that is, employment of a twin-screw vent-type kneading extruder, and horizontal stirred polymerization tank, respectively. Further, Japanese Patent Application Laid-Open No. 175722/1990 discloses a process for controlling the content of a hydrolyzable chlorine compound in the monomer under a prescribed level. Such problems, however, have not completely been dissolved, and a satisfactory PC has not been obtained yet.
Under these circumstances, the present inventors have made intensive research to improve transparency, tone of color, heat resistance and water reistance of PC and to develop a process for efficiently producing PC.